Lignin is an abundant phenolic biopolymer, comprising approximately one quarter of the weight of dry wood. The structure of lignin is quite complex and insights into its structure have only recently been elucidated. As contrasted with other biopolymers such as starch, cellulose, proteins, nucleic acids and the like, lignin does not possess any repeating units nor are its chemical bonds particularly easy to hydrolyze. Ether bonds are often prevalent, providing a variety of linkages to the numerous aromatic residues. A portion of a typical structure is illustrated in FIG. 1.
Lignin is synthesized non-enzymatically by the coupling of three types of phenylpropane units (coumarylalcohol, coniferylalcohol, and sinapylalcohol) to form dilignols, trilignols and oligolignols which continue to couple in a random fashion with each other and additional monomer units. The lignin product is a copolycondensate, insoluble in water and possessing a molecular weight of tens of thousands.
Although generally resistant to microbial attack, lignin degradation has been demonstrated among members of litter-decomposing fungi, most notably the so-called white-rot fungi. One particular species, Phanerochaete chrysosporium, has been the focus of a good deal of research (See for example: D. Ulmer et al., Eur. J. Appl. Microbiol. Biotechnol. 18: 153-157 (1983) and M. Leisola, et al. Arch. of Microbiol 137: 171-175 (1984)). Additionally, lignin degradation has been shown to occur by the action of certain strains of Streptomyces spp. (T. Pettey et al., Appl. Environ. Microbiol. 47 (2): 439-440 (1984)) as well as by a bacterium tentatively identified as belonging to the genus Arthrobacter isolated from decaying peanut hulls (T. Kerr et al. Appl. Environ. Microbiol. 46 (5): 1201-1206 (1983)).
Recently, .sup.13 CNMR techniques have provided some insights as to mechanism of lignin degradation by white-rot fungi (M. Chua et al. Holzforschung 36: 165-172 (1982)). These studies indicate that aromatic nuclei integrated in the polymer are oxidatively cleaved with the aromatic moieties being further degraded to aliphatic structures.
It is desirable to identify new microbial strains capable of degrading lignin for use in the manufacture of of cellulosic products from lignocellulose by "biological pulping" as described in U.S. Pat. No. 3,962,033, the treatment of lignin-derived waste such as the Kraft black plant effluent (Kirk, T. et al., Biotechnol. Lett. 1: 347-352 (1979)), or the production of food or fuel (Lindenfelser, L. et al. Dev. Ind. Microbiol. 20: 541-551 (1979)). It is also desirable to isolate microoganisms capable of partial degradation of lignin in order to provide a source of valuable chemical feedstocks for the production of fine chemicals.